GLE and Sub-GLE Redefinition in the Light of High-Altitude Polar Neutron Monitors

• Published in: Solar physics Vol. 292; no. 11; pp. 1 - 7
• Main Authors: Poluianov, S. V., Usoskin, I. G., Mishev, A. L., Shea, M. A., Smart, D. F.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-11-2017; Springer Nature B.V
• Subjects: Altitude; Astrophysics and Astroparticles; Atmospheric Sciences; Elevation; Energetic particles; Geomagnetism; High altitude; Homogeneity; Light; Neutrons; Physics; Physics and Astronomy; Polar environments; Polar regions; Proton flux; Rigidity; Sea level; Solar energetic particles; Solar physics; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Energetic particles
• ISSN: 0038-0938; 1573-093X
• DOI: 10.1007/s11207-017-1202-4

The conventional definition of ground-level enhancement (GLE) events requires a detection of solar energetic particles (SEP) by at least two differently located neutron monitors. Some places are exceptionally well suitable for ground-based detection of SEP – high-elevation polar regions with negligible geomagnetic and reduced atmospheric energy/rigidity cutoffs. At present, there are two neutron-monitor stations in such locations on the Antarctic plateau: SOPO/SOPB (at Amundsen–Scott station, 2835 m elevation), and DOMC/DOMB (at Concordia station, 3233 m elevation). Since 2015, when the DOMC/DOMB station started continuous operation, a relatively weak SEP event that was not detected by sea-level neutron-monitor stations was registered by both SOPO/SOPB and DOMC/DOMB, and it was accordingly classified as a GLE. This would lead to a distortion of the homogeneity of the historic GLE list and the corresponding statistics. To address this issue, we propose to modify the GLE definition so that it maintains the homogeneity: A GLE event is registered when there are near-time coincident and statistically significant enhancements of the count rates of at least two differently located neutron monitors, including at least one neutron monitor near sea level and a corresponding enhancement in the proton flux measured by a space-borne instrument(s). Relatively weak SEP events registered only by high-altitude polar neutron monitors, but with no response from cosmic-ray stations at sea level, can be classified as sub-GLEs.